PROJECT SUMMARY We will advance the quantitative understanding of how human muscle structure impacts function, in health and disease; and we will share the magnetic resonance imaging (MRI) resources used to generate this knowledge freely with the muscle research community. At every level of biological complexity, muscle structure significantly influences muscle function. These properties include the intermediate-scale relationships known as muscle architecture: the shape and orientation of a muscle?s fibers with respect to its mechanical line of action. Our understanding of how muscle architecture affects muscle function remains incomplete, however, and our tools for studying these relationships are insufficiently developed. As a result, there are critical gaps in our understanding of how pathologically altered muscle architecture in diseases such as Duchenne muscular dystrophy (DMD) impairs in vivo, whole-muscle function and exacerbates these muscles? risk of further injury. To provide this knowledge, we will advance the technology and application of quantitative MRI techniques such as diffusion-tensor imaging (DTI), overcoming several remaining technical challenges and developing an improved understanding of muscle architecture and function. Aim 1 is to validate MRI methods for quantifying muscle architecture in a broad range of states of muscle health and disease, at rest and during contraction. We will validate DTI fiber-tracking algorithms for quantifying the architecture of healthy, atrophied, inflamed, and fat-infiltrated muscles, and we will develop and validate methods that combine DTI fiber-tracking and rapidly acquired 3D images to quantify muscle architecture during contraction. Aim 2 is to advance the quantitative understanding of the functional impact of muscle architecture in healthy and dystrophic human muscle. We will quantify the relationships among muscle architecture and force generation, strain development, and the sufficiency of peripheral oxygen supply. The outcome of this work will be a newly identified physiologic mechanism of injury in DMD and the scientific foundation for using advanced structural and functional MR imaging to evaluate and guide therapy. Aim 3 is distribute data and software for MRI-based muscle structure-function analysis. A whole-body imaging dataset will be made publically available. Also, a software toolkit for processing these data will be made freely available and supported through collaborations. Overall, we will develop optimal methods for analyzing DTI data from healthy and diseased muscles and integrating these data with those available from other MRI sequences. We will create new knowledge about the relationships between muscle structure and function and how they are impacted by disease. By developing these advances into a freely available dataset and toolkit, we will enable musculoskeletal researchers worldwide to apply these methods in applied physiology and translational studies.